The modulator in a transmitter in a mobile radio is used for transformation of the binary data stream to a modulated carrier signal. For this purpose, the binary data stream is first of all converted to complex-value digital data symbols as a function of the modulation type. The data symbols are then subjected to pulse shaping at the transmission end, in order to increase the spectral efficiency. The resultant baseband signal is then mixed with a carrier.
The modulator may in this case be in the form of an I/Q modulator or a polar modulator.
In the case of an I/Q modulator, the complex baseband signal is mixed in a Cartesian representation, separately on the basis of the real part and the imaginary part, with two orthogonal carrier signals. The real part and the imaginary part are also referred to as the I component (in phase) and the Q component (quadrature phase). The two resultant radio-frequency signals are then added and amplified. An I/Q modulator has the disadvantage in comparison to the polar modulator described in the following text that its power consumption is higher and it occupies more area on a chip.
In the case of a polar modulator, the complex baseband signal in a polar representation is processed separately on the basis of the amplitude and phase. FIG. 1 shows the outline circuit diagram of a polar modulator. A binary data stream b is first of all converted by means of a symbol mapper 1 to a complex-value symbol sequence a(k). The complex-value symbol sequence a(k) in this case comprises an I component and an Q component. The complex-value symbol sequence a(k) is then converted by means of a complex pulse shaping filter 2 to a complex baseband signal x(k)=i(k)+jq(k). A digital computation circuit 3 is used to transform the complex baseband signal x(k) in a Cartesian representation to a corresponding signal x(k)=r(k)·ejφ(k) in polar representation. In this case, the variable r(k) describes an amplitude signal and the variable (φ(k) a phase signal.
An analogue carrier signal y(t)˜cos(ωc·t+Φ(t)) which is modulated as a function of the phase signal φ(k), is generated from the digital phase signal φ(k) by means of a step-up converter 4 which is based on a PLL (phase locked loop), in particular a directly modulated PLL. In this case, the variable ω0 corresponds to the carrier circular frequency. The phase Φ(t) is dependent on the phase signal φ(k) and the modulation type, for example phase modulation or frequency modulation). The digital amplitude signal r(k) is converted by a digital/analogue converter 5 to an analogue signal, which is then filtered by means of a noise filter 6 in order to reduce the quantization noise, with an analogue amplitude signal r(t) being produced. The analogue carrier signal y(t) and the analogue amplitude signal r(t) are multiplied by means of a multiplier 7. The multiplier 7 thus amplitude-modulates the analogue carrier signal y(t) as a function of the analogue amplitude signal r(t). The resultant signal s(t)=r(t)·y(t) is amplified by a power amplifier (not illustrated) before being transmitted via the antenna.
One embodiment of a polar modulator is illustrated in FIG. 2. Circuit parts and signals which are provided with the same reference symbols in FIG. 1 and FIG. 2 correspond to one another. In contrast to the polar modulator which is illustrated in FIG. 1, the amplitude modulation is in this case carried out only in the output stage of the power amplifier 8. For this purpose, the power gain of the power amplifier 8 is modulated via a modulation input 10 as a function of the analogue amplitude signal r(t). The supply voltage for the output stage of the power amplifier 8 is typically modulated in order to modulate the power gain. Alternatively, the quiescent currents of the output stage of the power amplifier, which govern the gain, may also be modulated. The supply voltage is modulated via an LDO voltage regulator (Low Dropout Regulator), which is not illustrated in FIG. 3. The polar modulator in FIG. 2 does not require the additional multiplier 7 as in FIG. 1.
One major advantage of the polar modulator illustrated in FIG. 2 over the polar modulator illustrated in FIG. 1 is that no linearity is required between the input signal y′(t) (y′(t) is generated by a limiting amplifier 9 from the signal y(t)) and the output signal from the power amplifier 8. The amplitude information r(k) in the symbols x(k) to be transmitted is not taken into account until the output stage, in any case. A polar modulator such as this operates particularly efficiently in power terms owing to the reduced linearity requirements on the power amplifier 8. The approach of not carrying out the amplitude modulation until the output stage of the power amplifier 8 is also referred to as the EER (Envelope Elimination and Restoration) method and is based on the publication “Single Sideband Transmission by Envelope Elimination and Restoration”, L. Kahn, Proceedings of I.R.E., July 1952, pages 803 to 806. The polar modulator illustrated in FIG. 2 has the disadvantage that the output signal s(t) is distorted by the power amplifier 8. The power amplifier 8, in particular the analogue circuit components which follow the modulation input 10 as well as the analogue circuit parts which drive the modulation input 10, thus causes amplitude distortion, which is also referred to as AM-AM distortion (AM=Amplitude Modulation), that is to say the relationship between the amplitude of the output signal s(t) and the amplitude signal r(k) is not linear. Furthermore, the power amplifier 8 causes phase distortion, which is also referred to as AM-PM distortion (PM=Phase Modulation), that is to say an additional phase shift as a function of the amplitude of the amplitude signal r(k).
The document “Polar Modulator for Multi-mode Cell Phones”, W. Sander et al., Proc. IEEE Custom Integrated Circuits Conf., September 2003, pages 439-445 discloses the digital amplitude signal r(k) and the digital phase signal φ(k) each being subjected to predistortion, so that the AM-AM distortion and the AM-PM distortion are compensated for. FIG. 3 illustrates a polar modulator which has had predistortion added to it, and is based on the polar modulator illustrated in FIG. 2. Circuit parts and signals which are provided with the same reference symbols in FIG. 2 and FIG. 3 correspond to one another. The polar modulator has a digital amplitude predistorter 11 and a digital phase predistorter 12, which compensate for the AM-AM distortion and the AM-PM distortion caused by the power amplifier 8.
The document U.S. Pat. No. 6,366,177 B1 describes an approach for adjustment of the predistortion. For this purpose, special measurement circuits are provided at the output of the power amplifier, which determine the amplitude and the phase modulation of the distorted radio-frequency signal s(t) with the phase modulation in each case fixed. The measured amplitude information and phase information are used to adjust the predistortion such that the distortion caused by the power amplifier is essentially compensated for. This approach has the disadvantage that the measurement circuit design involves a high degree of circuit complexity. In particular, step-down conversion of the radio-frequency signal s(t) to a baseband signal is first of all required, in order to determine the phase modulation.